5,542 research outputs found
Superconducting Detectors for Super Light Dark Matter
We propose and study a new class of superconducting detectors which are
sensitive to O(meV) electron recoils from dark matter-electron scattering. Such
devices could detect dark matter as light as the warm dark matter limit, mX >
keV. We compute the rate of dark matter scattering off of free electrons in a
(superconducting) metal, including the relevant Pauli blocking factors. We
demonstrate that classes of dark matter consistent with terrestrial and
cosmological/astrophysical constraints could be detected by such detectors with
a moderate size exposure.Comment: 6 pages, 2 figures; v2: improved detection discussion, modified
benchmark mode
Photodetection in silicon beyond the band edge with surface states
Silicon is an extremely attractive material platform for integrated optics at
telecommunications wavelengths, particularly for integration with CMOS
circuits. Developing detectors and electrically pumped lasers at telecom
wavelengths are the two main technological hurdles before silicon can become a
comprehensive platform for integrated optics. We report on the generation of
free carriers in unimplanted SOI ridge waveguides, which we attribute to
surface state absorption. By electrically contacting the waveguides, a
photodetector with a responsivity of 36 mA/W and quantum efficiency of 2.8% is
demonstrated. The photoconductive effect is shown to have minimal falloff at
speeds of up to 60 Mhz
Low-energy electron beam focusing in self-organized porous alumina vacuum windows
Micromachined, micron-thick porous alumina membranes with closed pore endings show high electron transparency above an energy of 5 keV. This is due to the channeling of electrons along the negatively charged insulating pores after surmounting the thin entrance layer. We also find a sharp hightransparency energy window at energies as low as 2 keV which may be the result of a local maximum of channeling, as predicted by simulations, and positive charge up of the entrance layer causing electron electrostatic focusing. Applications for these membranes range from atmospheric electron spectroscopy to self-assembled, nanoscale, large-area electron collimators
Design of a tunable, room temperature, continuous-wave terahertz source and detector using silicon waveguides
We describe the design of a silicon-based source for radiation in the 0.5-14 THz regime. This new class of devices will permit continuously tunable, milliwatt scale, cw, room temperature operation, a substantial advance over currently available technologies. Our silicon terahertz generator consists of a silicon waveguide for near-infrared radiation, contained within a metal waveguide for terahertz radiation. A nonlinear polymer cladding permits two near-infrared lasers to mix, and through difference-frequency generation produces terahertz output. The small dimensions of the design greatly increase the optical fields, enhancing the nonlinear effect. The design can also be used to detect terahertz radiation
Detecting Superlight Dark Matter with Fermi-Degenerate Materials
We examine in greater detail the recent proposal of using superconductors for
detecting dark matter as light as the warm dark matter limit of O(keV).
Detection of such light dark matter is possible if the entire kinetic energy of
the dark matter is extracted in the scattering, and if the experiment is
sensitive to O(meV) energy depositions. This is the case for Fermi-degenerate
materials in which the Fermi velocity exceeds the dark matter velocity
dispersion in the Milky Way of ~10^-3. We focus on a concrete experimental
proposal using a superconducting target with a transition edge sensor in order
to detect the small energy deposits from the dark matter scatterings.
Considering a wide variety of constraints, from dark matter self-interactions
to the cosmic microwave background, we show that models consistent with
cosmological/astrophysical and terrestrial constraints are observable with such
detectors. A wider range of viable models with dark matter mass below an MeV is
available if dark matter or mediator properties (such as couplings or masses)
differ at BBN epoch or in stellar interiors from those in superconductors. We
also show that metal targets pay a strong in-medium suppression for kinetically
mixed mediators; this suppression is alleviated with insulating targets.Comment: 40 pages, 10 figures; v2: updated figures, matches published versio
Tolman wormholes violate the strong energy condition
For an arbitrary Tolman wormhole, unconstrained by symmetry, we shall define
the bounce in terms of a three-dimensional edgeless achronal spacelike
hypersurface of minimal volume. (Zero trace for the extrinsic curvature plus a
"flare-out" condition.) This enables us to severely constrain the geometry of
spacetime at and near the bounce and to derive general theorems regarding
violations of the energy conditions--theorems that do not involve geodesic
averaging but nevertheless apply to situations much more general than the
highly symmetric FRW-based subclass of Tolman wormholes. [For example: even
under the mildest of hypotheses, the strong energy condition (SEC) must be
violated.] Alternatively, one can dispense with the minimal volume condition
and define a generic bounce entirely in terms of the motion of test particles
(future-pointing timelike geodesics), by looking at the expansion of their
timelike geodesic congruences. One re-confirms that the SEC must be violated at
or near the bounce. In contrast, it is easy to arrange for all the other
standard energy conditions to be satisfied.Comment: 8 pages, ReV-TeX 3.
Spatio-temporal patterns driven by autocatalytic internal reaction noise
The influence that intrinsic local density fluctuations can have on solutions
of mean-field reaction-diffusion models is investigated numerically by means of
the spatial patterns arising from two species that react and diffuse in the
presence of strong internal reaction noise. The dynamics of the Gray-Scott (GS)
model with constant external source is first cast in terms of a continuum field
theory representing the corresponding master equation. We then derive a
Langevin description of the field theory and use these stochastic differential
equations in our simulations. The nature of the multiplicative noise is
specified exactly without recourse to assumptions and turns out to be of the
same order as the reaction itself, and thus cannot be treated as a small
perturbation. Many of the complex patterns obtained in the absence of noise for
the GS model are completely obliterated by these strong internal fluctuations,
but we find novel spatial patterns induced by this reaction noise in regions of
parameter space that otherwise correspond to homogeneous solutions when
fluctuations are not included.Comment: 12 pages, 18 figure
Dynamic wormholes
A new framework is proposed for general dynamic wormholes, unifying them with
black holes. Both are generically defined locally by outer trapping horizons,
temporal for wormholes and spatial or null for black and white holes. Thus
wormhole horizons are two-way traversible, while black-hole and white-hole
horizons are only one-way traversible. It follows from the Einstein equation
that the null energy condition is violated everywhere on a generic wormhole
horizon. It is suggested that quantum inequalities constraining negative energy
break down at such horizons. Wormhole dynamics can be developed as for
black-hole dynamics, including a reversed second law and a first law involving
a definition of wormhole surface gravity. Since the causal nature of a horizon
can change, being spatial under positive energy and temporal under sufficient
negative energy, black holes and wormholes are interconvertible. In particular,
if a wormhole's negative-energy source fails, it may collapse into a black
hole. Conversely, irradiating a black-hole horizon with negative energy could
convert it into a wormhole horizon. This also suggests a possible final state
of black-hole evaporation: a stationary wormhole. The new framework allows a
fully dynamical description of the operation of a wormhole for practical
transport, including the back-reaction of the transported matter on the
wormhole. As an example of a matter model, a Klein-Gordon field with negative
gravitational coupling is a source for a static wormhole of Morris & Thorne.Comment: 5 revtex pages, 4 eps figures. Minor change which did not reach
publisher
Wormhole Cosmology and the Horizon Problem
We construct an explicit class of dynamic lorentzian wormholes connecting
Friedmann-Robertson-Walker (FRW) spacetimes. These wormholes can allow two-way
transmission of signals between spatially separated regions of spacetime and
could permit such regions to come into thermal contact. The cosmology of a
network of early Universe wormholes is discussed.Comment: 13 pages, in RevTe
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